Organised expectation-management has become the politics of the climate talks in Cancún.

"Don't expect too much," officials brief, while keeping their bosses out of sight. But, applying to dangerous global warming the same diplomatic took-kit used for thousands of other negotiations is to make what is called a "category error". Politicians are not alone. The routine campaigning of the big green and anti-poverty groups similarly drifts in expectation of disappointment from one UN conference to another, scoring a few moral points along the way.

But the situation now is different. The category error is to treat climate negotiations like any other in which the outcome sits on a scale of better or worse. Normally its theoretically possible to seek a compromise that pleases, or at least doesn't disappoint, all sides. You can argue for more or less poor country debt relief, for better terms of trade, more funding of sport or the arts, and an incrementally better health and education service. But, more like avoiding a nuclear holocaust, avoiding runaway global warming is a game of absolute avoidance.

The prospect for global temperature rises of 4C are explored in a new collection of research papers published by the Royal Society. But, in one paper, according to Kevin Anderson and Alice Bows of the Tyndall Centre for Climate Research at the University of Manchester, the impacts related to a rise of just half that are so bad, "that 2C now represents the threshold [of] extremely dangerous climate change".

This is the problem. Once the planet warms to the point where environmental changes that further add to warming feed off each other, it becomes almost meaningless to specify just how much warmer the planet may get. You've toppled the first domino and it becomes virtually impossible to stop the following chain of events. Honestly, nobody really knows exactly where that will end, but they do know it will end very, very badly.

If that is the danger, the whole game is to stop the first domino falling. It is not to lean back in conference chairs and complacently discuss how much ground your delegation is prepared to give, how much you are prepared to spend and what your "best offer" is. As Winston Churchill once said, the point is not that we must do our best, we have to succeed in doing what is necessary.

But what does that kind of organised, international political process look like? Well, it looks a bit like what governments have done over the past three years to preserve, at all costs (and those costs have been very high) the functioning of the international banking system. The question was asked: what is the scale of action necessary to avoid financial collapse? Talks were had, feet were stamped, tears were shed and the money and measures were found. If they hadn't been, we would all have been struggling to get by in economies rather like Argentina's after the 2001 run on the banks. We would have got by, with difficulty but we would have survived.

The same question about the scale of action necessary to avoid crossing the 2C threshold is not, honestly, being asked by the governments we trust to protect us from harm. There are plenty of talks, but the money and measures are not being found.

There is a massive gap between rich countries' pledges to reduce emissions and what the science says is needed.

Renewable energy targets in Britain are not being met, according to the public accounts committee. And, you can find editorials in City AM, the daily paper of the City of London, attacking the "green consensus" of the three main parties and calling for the government's policies on carbon reductions to be "loosened". Category error: avoiding the loss of a habitable climate is far less negotiable than maintaining the banking system.

A recent UN report said that conventional economic development in Africa was creating "oceans of poverty and islands of wealth". If the worst case scenario in the Royal Society papers spoke of a 4C temperature rise by 2060 happens, our legacy will be just that, oceans and islands.

Action to prevent dangerous climate change needs to be put in a new category.